1. Field of the Invention
The present invention relates to an electric double layer capacitor having a large capacity used for power devices or electronic circuits. In particularly, the present invention relates to an electric double layer capacitor used as a unit capacitor for forming an assembled cell structure by connecting a plurality of unit capacitors in series by means of outer lead wires and an assembled cell type power source device for outputting a voltage of 8 V or more, which is formed by connecting a plurality of the unit capacitors in series.
2. Description of the Related Art
From the standpoint of utilizing effectively energy, electric double layer capacitors having a large capacity for power devices have widely been applied to, for example, a hybrid automobile mounting thereon a power source system constituted by a gasoline engine/electric double layer capacitor, an electric automobile, a power source system of a solar cell/electric double layer capacitor. Further, needs for a power source for electronic devices using the electric double layer capacitor has also been increasing. As the electric double layer capacitor having a large capacity for power devices, there have been developed one which employs an aqueous solution type electrolyte and one which employs an organic solvent type electrolyte. Since the electric double layer capacitor of low resistance and large capacity which employs an organic electrolyte can increase the working voltage, it can increase energy density in comparison with the electric double layer capacitor employing an aqueous solution type electrolyte.
In the electric double layer capacitor employing an organic electrolyte, a capacitor element is formed by rolling a pair of opposing electrodes interposing a separator therebetween wherein each of the electrodes comprises a metallic current collector foil on which a thin film polarization electrode composed mainly of activated carbon powder is attached. The capacitor element which is impregnated with an organic electrolyte is put in a bottomed cylindrical casing made of metal. The opening of the metallic casing is sealed with a sealing material such as a sealing rubber, gasket, a Bakelite plate lined with rubber or a lamination plate of phenol resin and rubber to prevent the electrolyte from evaporation. Further, there has recently been proposed to use a polymer electrolyte gel.
Besides of the electric double layer capacitor of so-called the electrode foil rolling type, Japanese Unexamined Patent Publication JP-A-4-154106 proposes an electric double layer capacitor of large current and capacity in which a lamination type capacitor element is installed, which is formed by stacking a large number of electrodes and separators in a flat plate shape. The proposed capacitor has a large capacitance and is durable to a large current.
The lamination type electric double layer capacitor is formed as follows. The capacitor element is formed by stacking a large number of rectangular polarization electrodes and rectangular separators wherein the separators are alternately inserted between the polarization electrodes. A positive electrode and a negative electrode are connected respectively with a lead wire for the positive electrode and a lead wire for the negative electrode at each end portion by crimping operations. The capacitor element with the lead wires is put in a bottomed polygonal tubular casing made of aluminum. An organic electrolyte is impregnated into the capacitor element. Then, the opening of the aluminum casing is sealed with a cover plate of aluminum.
For the electrodes, either of the positive electrode or the negative electrode for the electric double layer capacitor is composed mainly of activated carbon or polyacene having a large specific surface area. Recently, the electric double layer capacitor for power devices as a unit cell having an electrostatic capacitance of 10-10,000 F and a working voltage of 2.3-3 V has been developed. In a case of using the electric double layer capacitors for power devices, a plurality of, e.g., a 4 to 160 number of unit capacitors are connected in series by means of outer lead wires to obtain a predetermined power source voltage, such as d.c. 8 V to 380 V.
The electric double layer capacitor formed by connecting a plurality of unit capacitors in series had the problem as follows. When a voltage is applied to the electric double layer capacitor, the voltage was not uniformly applied to the unit capacitors due to a scatter in leakage current in the unit capacitors and a voltage beyond the working voltage is applied to some unit capacitors, with the result that there took place deterioration in the capacitors which invited the deterioration of the entire unit capacitors connected in series.
For example, when a voltage which is 0.15 V or more higher than the working voltage is applied to a unit capacitor for a long term, the deterioration of the capacitor is remarkable. Accordingly, it is necessary to determine the upper limit of a voltage applied to each unit capacitor in an assembled cell structure, specifically, it is necessary to determine the upper limit of voltage in a manner that 0.15 V, preferably 0.10 V is added to the working voltage.
Further, since the working voltage of the unit capacitor element in the electric double layer capacitor employing an aqueous solution type electrolyte is 0.8-1 V, a laminated capacitor having a rated voltage of 8-200 V is formed by stacking 10-200 unit elements. However, such lamination type capacitor also has a problem of scattering in voltage in the same manner as above mentioned.
In order to eliminate such problem, there has been known a method of connecting resistors each having the same resistance in parallel to the serially connected unit capacitors to thereby reduce a scattering in voltage in each of the serially connected unit capacitors (first conventional technique). Further, Japanese Unexamined Patent Publication JP-A-6-302474 proposes a method wherein in a lamination type electric double layer capacitor in which a plurality of unit capacitors are directly stacked without using an outer lead wire, a protective circuit comprising a serial connection of a Zener diode and a resistor is connected in parallel to each of the unit capacitors (second conventional technique).
However, in the first conventional technique wherein the resistor is connected in parallel to each of the unit capacitors, an amount of leakage current increases to 5-10 times due to the resistors connected in parallel whereby an amount of self-discharging becomes large, and the voltage holding characteristics is remarkably reduced.
In the second conventional technique wherein the Zener diode and the resistor are used, there was found a certain effect in an electric double layer capacitor employing an aqueous solution type electrolyte wherein the electrostatic capacitance of the element was small as about several farads. However, when the electrostatic capacitor of the unit element was large as several tens farads and the number of stacked unit elements was large as 10-300 layers, a scatter in voltage in the unit elements became large.
Further, in the electric double layer capacitor employing an organic type electrolyte wherein the capacitance was large as several tens farads or more, little effect could be expected in equalization of voltage, and further, it was difficult to assure a sealing function at the side surface of the stacked unit capacitors. Further, the working voltage of the capacitors was decreased due to permeation of water from the outside.
Description is made with reference to FIG. 5 showing the operational characteristic diagram of Zener diode. In the conventional technique, a Zener diode was used in a portion Eb where a current in the Zener diode rised. On the other hand, the inventors of this application have noted a portion where a current in the Zener diode sharply rises, and have extensively studied on a method capable of reducing a scattering in an applied voltage while occurrence of short circuit in an over voltage is suppressed. As a result, they have found to reduce substantially a scattering of applied voltage by combining a specified Zener diode characteristic with a capacitor characteristic. Further, they have succeeded to suppress a scattering of voltage by arranging Zener diodes and fuses while safety in a protective circuit is improved without deteriorating the voltage holding characteristics.